Twin-field (TF) quantum key distribution (QKD) has emerged as a promising method for repeater-free long-distance secure communication, with the longest demonstration covering 1000 km [1]. It is a measurement-device-independent QKD protocol, where the two users send weak coherent states, to a central node for single-photon interference followed by single-photon detection. Due to this, TF-QKD has a square root dependence of the key rate on channel transmission, making it possible to cover longer distances than conventional or commercial devices[2]. In previous implementations, a narrow linewidth laser has been used as the master or reference laser, frequency stabilised by locking the laser to a frequency reference, such as the atomic transition of acetylene or an ultra-low expansion (ULE) cavity using RF-locking [1], to achieve high coherence throughout the entire system. The RF-locking method requires a large and complex infrastructure that is sensitive to misalignment and is relatively expensive, bulky, and fragile [3]. Here, we demonstrate an all-fibre frequency reference as an alternative to ULE cavities for TF-QKD, offering lower complexity and intrinsic alignment, a simplified setup built with telecom compatible components, and a smaller footprint, which makes it easier to deploy in existing fibre telecom infrastructures.

Two-dimensional periodically poled LiNbO₃ and LiTaO₃ lattices afford unique spectral and spatial functionalities, enabling coherent parametric sources and amplifiers for advanced classical and quantum applications [1,2]. Such capabilities can be further enhanced with structured optical excitations, as highlighted in recent theoretical studies on dual-pump optical parametric generation (OPG) [3]. Here we present results confirming such predictions with experiments in hexagonally poled LiTaO₃ (HexLT) coherently excited by a dual-beam pump at P = 532 nm (Fig.1a, wavevectors k_p1 and k_p2), generating signal (k_s) and idler (k_i) outputs around s ∼ 0.76-0.82 μm and i ∼ 1.50-1.77 μm, respectively. Moreover, we model the OPG process in a semiclassical approximation and 2D beam propagation, extending the results of Ref. [3] beyond plane-wave approximations, to account for realistic experimental conditions. Fig. 1b shows a 2D map of the computed spectral (s) and angular (θs) signal emission from a HexLT lattice pumped by two Gaussian beams whose incidence angles realize the special pump-lattice resonance condition whereby the transverse components of k_p1 and k_p2 match those of reciprocal lattice vectors (G₁₀ and G₀₁ in Fig. 1a). The OPG simulations exhibited excellent agreement with the measured far-field distributions (Fig. 1c) and supported further quantitative analyses of the gain enhancement and coherent OPG response expected in the spectral region of Fig. 1d, explored in subsequent experiments with the setup of Fig. 1e. Fig. 1f shows experimental results (blue circles) alongside simulations (red triangles) and experimental fits (dashed lines), providing evidence for the periodic phase-sensitive response and coherent parametric gain enhancement over single-pump OPG, attainable through control of the relative phase of the two pumps in the HexLT [3]. Further experimental mappings of the signal versus total pump power in dual- and single-pump cases yielded excellent quantitative agreement with the results of our numerical model, resulting in a power amplification factor of around 1.1, matching well the measured visibility seen in the phase-sensitive response of Fig. 1f.

Counterpropagating (CP) χ⁽²⁾ interactions have since the inception of nonlinear optics garnered significant interest due to the unique features of their wave-dynamics and spectral response, stemming from their inherent feedback. The extremely short periods required for their quasi-phase matching (QPM) imply significant technology challenges, that are nevertheless being overcome by advances in periodically poled (PP) materials, with most prominent results achieved in bulk PPKTP under pulsed excitations [1]. Very recently, continuous-wave (CW) operation in ultralow-footprint devices has been achieved on the emerging periodically poled thin film LiNbO₃ (PPTFLN) platform, with symmetric second harmonic generation (SHG) in Z-cut waveguides [2]. Here we present the first results on CP frequency conversion in X-cut PPTFLN, the most widely used cut for photonic integrated circuits in LN [3], which has however proven harder to pole with short periods and high uniformity. At difference to the work of [2], we extensively explore the spectral response in sum frequency generation (SFG) experiments, considering nondegenerate and asymmetric regimes, which provide efficient unidirectional emission.

Squeezed states of light produced by nonlinear interactions in periodically poled crystals are specially engineered quantum states where the uncertainties in the phase or amplitude quadrature can be traded against each other. These low-noise states have applications in quantum communication, sensing, and computing [1]. To fully exploit these applications, precise phase locking of the squeezed light to a local oscillator (LO) phase is essential. For single-pass waveguides the used locking technique has relied on the injection of a frequency-shifted input seed to generate the error signal [1]. Here we assess the advantages and challenges of implementing a technique originally devised for cavity-based squeezed light sources called quantum noise locking (QNL) [2], which features a simplified feedback structure, eliminating the need for an additional frequency shifted field, thereby avoiding seeding and extra optical components. This work experimentally extends QNL to periodically poled LiNbO₃ waveguides for the first time, which allows for phase controlled squeezing, with less than 10 mrad phase noise, over a 50 GHz bandwidth.

Coupled optical cavities, which support normal modes, play a critical role in optical filtering, sensing, slow-light generation, and quantum state manipulation. Recent theoretical work has proposed incorporating nonlinear materials into these systems to enable novel quantum technologies. Here, we report the first experimental demonstration of squeezing generated in a quantum-enhanced coupled-cavity system, achieving a quantum noise reduction of 3.3 dB around the normal-mode splitting frequency of 7.47 MHz. We provide a comprehensive analysis of the system's loss mechanisms and performance limitations, validating theoretical predictions. Our results underscore the promise of coupled-cavity squeezers for advanced quantum applications, including gravitational wave detection and precision sensing.

Phase-sensitive amplification of squeezed states is a technique to mitigate high detection loss, which is especially attractive at 2 mu m wavelengths. We derived an analytical model proving that amplified squeezed states can mitigate phase noise significantly. Our model discloses two practical parameters: the effective measurable squeezing and the effective detection efficiency of amplified squeezed states. A realistic case study includes the dynamics of the gain-dependent impedance matching conditions of the amplifier. Our results recommend operating the optical parametric amplifier at high gains because of the signal-to-noise ratio's robustness to phase noise. Amplified squeezed states are relevant in proposed gravitational wave detectors and interesting for applications in quantum systems degraded by the output coupling loss in optical waveguides.

Conventional heterodyne readout schemes are now under reconsideration due to the realization of techniques to evade its inherent 3 dB signal-to-noise penalty. The application of high-frequency, quadratureentangled, two-mode squeezed states can further improve the readout sensitivity of audio-band signals. In this Letter, we experimentally demonstrate quantum-enhanced heterodyne readout of two spatially distinct interferometers with direct optical signal combination, circumventing the 3 dB heterodyne signal-to-noise penalty. Applying a high-frequency, quadrature-entangled, two-mode squeezed state, we show further signalto-noise improvement of an injected audio band signal of 3.5 dB. This technique is applicable for quantumlimited high-precision experiments, with application to searches for quantum gravity, searches for dark matter, gravitational wave detection, and wavelength-multiplexed quantum communication.

The detection of kilohertz-band gravitational waves promises discoveries in astrophysics, exotic matter, and cosmology. To improve the kilohertz quantum noise-limited sensitivity of interferometric gravitational -wave detectors, we investigate nondegenerate internal squeezing: optical parametric oscillation inside the signal-recycling cavity with distinct signal-mode and idler-mode frequencies. We use an analytic Hamiltonian model to show that this stable, all-optical technique is tolerant to decoherence from optical detection loss and that it, with its optimal readout scheme, is feasible for broadband sensitivity enhancement.